In recent years, stricter legislations on exhaust emissions have emerged worldwide. Among these are restrictions on the amount of particulate matter (soot) that the vehicle may produce. A solution to this problem is the use of a Particulate Filter (PF) in the exhaust stream of a diesel or gasoline engine. A typical PF is a cylinder shaped ceramic (silicon carbide or cordierite) body: a honeycomb structure with lots of different parallel channels. Adjacent channels are closed off at each end by plugs to force the exhaust gas to penetrate through the walls. Soot from the exhaust will accumulate on these walls. From time to time this accumulated soot needs to be burned off, the so-called regeneration stage. During regeneration, the exhaust temperature is artificially elevated, for example by a secondary injection of diesel or retarded main injection. The regeneration is mainly managed automatically by the vehicle's ECU. In theory, this process unclogs the filter by burning off the soot into CO and CO2. In practice, no burning process is perfect (especially on vehicles that only make short trips, due to the lower exhaust temperatures) and ash is formed and builds up in the filter as well (during regeneration, but also during normal operation). This ash is not removed by regeneration (as ash is already burned), and after some time the back-pressure over the PF, caused by the ash build-up, will become too high for the engine to overcome. At that point, the DPF needs to be replaced or cleaned. In Europe, the emission regulations have only recently become so strict that a PF has become necessary. In the US however, due to regulations for heavy transport (trucks . . . ), PF's are commonplace. The US market therefore has developed several means to clean out these filters, as replacement is a costly option.
Two main cleaning approaches are identified: the chemical or wet approach and the physical or dry approach. In the wet approach, a cleaning fluid (mostly water based) under pressure and sometimes warmed, is introduced into the filter and forced through the channels to clean out the ash and soot particles. The fluid is sometimes mixed with pressurised gas to form bubbles to improve cleaning. Ultrasonic waves are in some cases introduced into the liquid as well to improve cleaning. A critical step in this process however is the final drying of the filter internals with (warm) air. Any remaining moisture in the filter can create violent steam expansions, as the exhaust temperatures are in the hundreds of degrees centigrade: in these conditions, water will instantly turn to steam. These expansions are able to irreparably damage the filter, thus a major weak point for wet cleaning. A second remark is the fact that under the EURO 6 standard, particle filters are coated with a catalyst to increase the efficiency. This coating can be damaged by some cleaning liquids, so special fluids are needed. Lastly, the used fluid needs to be disposed of in an environmentally sound way, or filtered to be used again. Chemical cleaning is because of these reasons not desired.
The developed physical processes however only use the properties the filter was designed for in the first place: pressure and air flow. Accumulated particles are removed by a blast of pressurised air (air pulse cleaning). The dislodged particles are subsequently blown off by air flow. No foreign matter is therefore introduced in the filter. Other dry processes use moving air nozzles directed towards the filter openings to blow out any blockages, see e.g. WO 201175598. Lastly, sometimes mechanical vibrations are employed to dislodge particulate matter, e.g. patent DE 102004029640, and WO2011156477. The Japanese reference JP H08 177 462, uses the combination of vibrations and a stream of air, but such that a stream of pure air is applied as a counter current to a stream of dirty air leaving the filter, and wherein during the adjustment of this counter current of pure air a shock or vibration of the filter is generated. These vibrations are produced by an unbalanced motor, but are not claimed to be related to any resonance property. Only ‘in- and decreasing frequency’ is mentioned, order of magnitude of 100 Hz and 10 mm for 5-10 min. This process is however also linked to chemical cleaning. Patent EP 1162351 from PSA also applies mechanical vibrations, but in this patent the vibrations originate from the ICE itself, during operation. Instead of mechanical vibrations in these dry cleaning methods acoustic waves may equally be employed (see US2007/0137150), but as in the foregoing cases, the application of vibrations is always part of a process step in which a flow of gas running through the assembly.
The main problem with the dry approaches, is that they are designed with big truck PF's in mind. These truck PF's are essentially just large ceramic cylinders and are therefore easy to blow air onto: the entire top and bottom area of the filter are accessible. The problem with the particulate filters mounted on cars is that they are contained in the exhaust tubes. The filter ceramic body cannot easily be removed from the tube. The exhaust tube containing the filter usually has pipes connected to the entrance and exits. These entrances and exits are also narrower than the filter body. All these properties complicate the dry cleaning process, as the air pressure will not be able to evenly strike the filter body. Some solutions have been proposed based on existing truck PF cleaning, for example by successively introducing cones and rings into the filter tube, to selectively apply air pressure to part of the filter. These accessory tools however need to be constructed for every filter size, and for some more complex filter shapes the proposed tools will not be able to be introduced. Also, they are easily lost and can be forgotten to take out of the tube after cleaning, blocking the filter. Manual labour time will also increase. Other proposed solutions are lowering a nozzle into the tube and rotating this around the filter body surface, applying an airflow to each part of the filter. This however requires either a straight entrance to the filter or a complex mechanism and a measurement system to determine the distance to the filter body, to avoid damage. Introducing nozzles is therefore quite a complicated method, due to the different forms and sizes of car DPF's.
Taking into account the weak points of wet systems, the preferred embodiment of the solution to car DPF cleaning is a dry process. Looking at the existing ‘solutions’ to the aforementioned problems, it is clear that these are rather complex, in comparison to truck DPF cleaning, or inelegant and requiring different extra tools. Keeping the system simple for the operator is however desired, e.g. cost-effective wise.